Temperature-controlled oven

ABSTRACT

Inherent heating of the air in an oven by a high-velocity recirculating blower is used as the sole heat source. Constant temperature is maintained by varying the rate of blending of ambient air with the heated oven air. Automatic control of temperature is provided by a variable vent operated by a thermal expansion bellows. The bellows is slightly responsive to ambient temperature to compensate for variations thereof.

United States Patent [1 1 Lawson Lawler Apr. 30, 1974 [54]TEMPERATURE-CONTROLLED OVEN 3,481,322 1,2/1969 Hokanson et a1 126/247Inventor: J p A e Flossmoor, m- 3,548,l53 12/1970 Kells 126/21 A [73]Assignee: Blue M Electric Company, Blue Primary Examiner-William F. ODeaIsland, 111. Assistant ExaminerPeter D. Ferguson [22] Filed: Feb 4, 1972gttorgey, Agent, or FirmBurmeister, Palmatier &

am [21] Appl. No.: 223,505 y a 57 ABSTRACT (g1. l26/v247fi2lfc6/l251/31;Inherent heating of the air in an oven y a g [58] Fieid 47. 122/26velocity recirculating blower is used as the sole heat 432/94 source.Constant temperature is maintained by varying the rate of blending ofambient air with the heated oven air. Automatic control of temperatureis pro- [56] References cued vided by a variable vent operated by athermal expan- UNITED STATES PATENTS sion bellows. The bellows isslightly responsive to am- 1,564,566 121925 Harris 34/225 X bienttemperature to compensate for variations 2,688,808 9 1954 lpsen 126/21 Ath fi 3,245,399 4/1966 Lawson.... 126/247 3,297,019 l/1967 126/247 6Claims, 6 Drawing Figures QPATENTEBAPR 30 m4 .3

sum 1 or 3 PATENTED PR 30 I974 sum 3 or 3 TEMPERATURE-CONTROLLED OVENThis invention relates to temperature-controlled ovens, particularly toovens for use with substances presenting hazards or explosion and thelike.

Severe problems are encountered in the employment of ordinary ovenconstructions in numerous applications presenting explosion and similarhazards. As a typical example, there may be considered the problemsencountered in use of an oven in precision controlledtemperature agingor similar long-term processing, or storage, of substances emittingexplosive fumes or vapors. For a number of reasons now to be discussed,it has long since been found that ovens for such purposes have highlyspecialized requirements.

A fairly obvious and common requirement for an oven for chemicalprocessing and similar uses, particularly where vapors or gases areemitted by the heating load, is a substantial rate of internal aircirculation. (It

will be understood, of course, that the term air is herein used forconvenience, this being the normal atmospheric environment inside andoutside an oven, the inclusion in this term of other gaseousenvironments occasionally employed being understood in connection withthe present invention). Were such circulation not provided, the heatenergy absorbed (or in some cases generated) in the reaction which isthe purpose of the processing would create local temperatures in theregion of the heating-load members defeating the basic purpose of thetemperature control, in addition to other undesired effects. Thecirculation also serves a substantial purpose in connection with theexplosion hazard, where such exists, since it prevents the accumulationof relatively undiluted hazardous vapors or gases. However where thedilution is not in itself sufficient to prevent an explosion hasard,i.e., where the concentration of the hazardous substance in the oven airis sufficient so that the mixture can explode or ignite, it becomeshighly necessary to assure against the existance of hot spots to whichthe oven air may find access, in addition to the obvious requirement ofbeing wholly assured against sparks and the like.

The maintenance of any given work-chamber temperature without having anyportion of the interior of the oven at a considerably higher temperatureis not a simple matter, and relatively complex and expensive structureshave often been provided for minimizing the temperature differentialbetween the work-chamber and other parts of the oven interior. As in anyoven wherein uniformity of temperature in the workchamber is animportant characteristic, the heating of the oven air is normally doneexternally of the workchamber in a suitable part of the oven enclosureforming a recirculation or air-return loop between the airoutlet andair-inlet portions of the workchamber, the blower also normally beingdisposed here. With an explosion hazard present, however, this heatingmay not be done with exposed heating elements or other comparably simplestructure, and substantial complication is often introduced in reducingthe maximum temperature to which the oven air is exposed. In addition,of course, the precautions against sparking, etc., are of similarly highrequirement.

Oven systems for such purposes must ordinarily be provided with someform of venting to constantly intro duce fresh air and at the same timeexhaust a fraction of the recirculating air, such provision beingrequired to prevent excessive build-up of reaction products in the ovenaside from explosion hazard. The explosion problem can of course beavoided despite the presence of high-temperature zones like heaterelements, etc., if the rate of bleeding-in of fresh air is sufficientlyhigh so that a mixture of reaction products and air is never adequatelyrich to be ignited. Thus one known alternative for complexity ofconstruction is substantial abandonment of the recirculation of ovenair, fresh air being constantly drawn in from the exterior, heated tothe requisite temperature, flushed through the workchamber and thereuponexhausted with the reaction products. Such a once-through systemrequires relatively large heating elements and heating power even forrelatively low temperatures, in addition to the fact that closeness ofcontrol of temperature, and uniformity of the temperature throughout thevolume, are more difficult to obtain than in the case of recirculatedair maintained within the enclosure.

A further, but related, difiiculty heretofore encoun-.

tered in controlled-temperature ovens for these and similar purposes isinability, in any simple manner, to achieve the maintenance ofrelatively low temperatures of the recirculated air. Where thetemperature is controlled by varying the power to heating elements,whether in proportional control or on-off cycle control, effectivecontrol cannot be obtained in the temperature region where the totalrequired heat input rate is of the same order as the variation of theheat-transfer variables, such as reaction rates in work-pieces, whichproduce temperature variations for which automatic compensation is to beeffected. Where a high-velocity blower is employed for circulation, itis found that control by heater-power variation becomes erratic andineffective at temperatures considerably higher than ambienttemperatures due to air-heating by the blower. To deal with this problemwhere temperatures only moderately above ambient temperature are wanted,various devices have heretofore been used such as the addition of arefrigerating unit in addition to a heater unit, thus extending thelower end of the temperature control range.

The present invention flows from recognition that the heating of the airproduced by blower operation, heretofore occasionally observed merely asa problem requiring complication in achieving relatively low controltemperatures, has unique advantages as opposed to other methods ofheating when itself employed as the sole heat source in an oven forpurposes such as those above described, particularly in applicationspresenting safety hazards. Most methods of heating of air, such as byresistance elements and the like, employ the physical phenomenongenerally known as heat transfer, which requires that the body orelement which heats the air be at substantially higher temperature thanthe air being heated. Where, in accordance with the present invention,the heating of the air is accomplished by friction and pressurephenomena such as those occurring at the blades of a blower, thenecessity for such a differential does not exist. Thus the air itselfcan be made the highest temperature portion of the system. With thispossibility, obtained from the nature of the basic process by which heatis imparted to the air, elaborate provisions for assuring against hotspots are eliminated without impairing the safety of permittingrelatively high concentrations of hazardous reaction products to bepresent in the recirculated air. This in turn permits employment of arelatively small fresh-air bleeding-rate and thus, in addition, raisesthe temperature to which the air can be heated by any given blower andmakes it practical to employ the blower as the sole heat power for asubstantial range of superambient oven temperatures.

In theory, the temperature of an oven employing the inherent blowerheating as the sole heat source might be adjusted and controlled byvarying blower speed or some similar parameter. A simpler and moreeconomical, as well as more accurate, control is achieved in accordancewith the invention by employing a constantspeed blower and varying therate of bleeding-in of ambient air (and of course bleeding-off of ovenair) in accordance with the oven temperature.

The invention, in implementation of the broader aspects discussed above,additionally provides novel construction features for accurate andsensitive variableventing temperature controls, for these and similarpurposes, wherein the vent remains closed until the preset temperatureis closely approached, whereupon the vent commences to open until theequilibrium temperature is reached. A simple type of temperaturetransducer is employed, with a relatively small, but neverthelessappreciable, exposure to ambient temperature, in addition to theexposure to oven temperature to which it is primarily responsive, sothat ambient temperature variations are minimized in effects on oventemperature.

The above aspects of the invention, as well as certain further ones,will best be understood by reference to the embodiments illustrated inthe drawing, in which:

FIG. 1 is a schematic sectional view of a temperaturecontrolled ovenincorporating the invention;

which is dependent on design of the particular blower l0 portion of theair-exit end of the enclosure and an employed, and may readily be madesufficiently small to make blade temperature a negligible factor in theoperation.

An air outlet vent 24 is provided through the upper air inlet vent 26through the lower portion. Slightly aboye inlet ent 26 is a vane ordamper 28 which forms an adjustableccii s ticmh in the air-flow pathbetween the exit end of the chamber 12 and the inlet duct 20 to theblower 18. Opening of such a damper to any predetermined degree hastheretofore been used to control the rate of replacement or renewal ofthe recirculating air. In employment of the present invention, thedamper 28, where one is provided, is placed in a predeterminedpartially-open fixed position to produce a small pressure differencebetween the inlet and outlet vented portions of the recirculationsystem.

'As earlier discussed, such an oven is conventionally automaticallycontrolled in temperature by varying the power supply to heatingelements. Although aspects of FIG. 2 is a more or less schematic view ofa temperature control assembly illustrative of the principle ofoperation of the temperature control of the oven of FIG. 1; FIG. 3 is anenlarged sectional view of the temperature control of the oven of FIG.1, taken along the line 3-3 of that Figure and of FIG. 4;

FIG. 4 is a sectional view taken along the line 44 of FIG. 3; y FIG. 5is a fragmentary enlarged view taken along the line 5-5 of FIG. 3; and

FIG. 6 is a fragmentary sectional view taken along the line 66 of FIG.4.

Except for the manner of heating and'maintaining fixed temperature, theoven of FIG. 1 is of conventional construction and thus is onlyschematically shown. The outer wall or enclosure 10 is of a usualthermalinsulated construction sealed against aipleakage except at thevents later mentioned. The work-chamber 12 (to which access is providedby the oven door, not shown) is formed of suitably perforated sheetmetalexcept for the imperforate floor portion 14, below which is a lowerinternal region 16 of the oven forming a return path for the generallyhorizontal circulation of air through the work-chamber 12. Aircirculation is provided by a high-velocity centrifugal blower 18associated with a sheet-metal duct or hood 20 which forms the air inletfor the blower, returning the workchamber exit air to the blower forrecirculation. The blower shaft 22 is externally driven by aconstant-speed motor below (not shown). The motor is external of theoven, which accordingly contains no electrical components.

If so desired, because of external explosion hazards the presentinvention to be later described may be employed for temperature controlas a substitute for such variation of power input where heating elementsare employed, the invention primarily. contemplates constructions whereno heating elements whatever are employed (or where heating elementsprovided for use at higher control temperatures are shut off). The soleheat provided is the heating of the recirculated air produced by theoperation of the high-capacity blower. Temperature control is effectedby automatic control of the venting, thus controlling the rate ofblending of ambient-temperature air with the recirculated air in theoven. For this purpose there is attached to the inlet vent at 26 acontrol mechanism responsive to the temperature sensed by the bulb 30 ofthe thermalexpansion temperature transducer having a capillary tube 32connected as hereinafter described to an external control assembly 34.

There is shown in FIG. 2 a control assembly 34a lacking certainrefinements of the assembly 34 shown screw mechanism 48.

The bottom of the housing 40 has transverse slots 50. slide-plate 52with slots 54 spaced in correspondence with the slots 50 abuts thebottom and is affixed to a suitable web or carriage 56, which slidesunder a shoe 6O 58 mounted on the carriage 44 by means of a leafspring60 which thus holds the slide-plate 52 in abutment against the bottom ofthe housing. A compression spring 62, acting between the housing and thecarriage 56, urges the latter to a limit position (leftward of thepartially open position shown in the drawing) wherein the slots 50 arewholly closed by the plate 52.

A lever or arm 63 has its upper end pivoted at 64 on -the carriage 44and has a transversely extending portion 66 in the path of motion of theplunger 38 on the bellows assembly. The lower end of the lever 62 isabutted by an extending portion 68 on the carriage 56 of theslide-plate.

The control assembly is illustrated in FIG. 2 in a partially opencondition of the venting slots representing an equilibrium oventemperature indicated by an appropriate calibration marking on theknob-and-dial 42. Any increase or decrease in temperature within theoven results in motion of the plunger 38 in the direction to increase ordecrease the admission of ambient air and thus compensate for whateverchange in conditions produced the change in oven temperature, such as athermal effect produced by the work-material being aged or otherwiseprocessed or stored. In start-up from a cold condition, the plunger 38is withdrawn and th slotted inlet is entirely closed, the lower end ofthe lever 62 being in the slightly clockwise position produced when thecarriage 56 and slide-plate 52 are at the limits of their leftwardmotion. The venting slots commence to open only when the presettemperature is approached, at which point the lever 62 is engaged by theplunger 38 which then further advances until the preset temperatureequilibrium is reached. By employment of the lever system with avariable vent producing an extremely large variation of resistance toair flow with small mechanical motion, there is achieved substantialaccuracy of maintenance of the preset temperature despite substantialvariations in, for example, exothermic or endothermic reaction rates inmaterials constituting the temperature-control load.

Variation of the preset temperature is accomplished by operation of thecontrol knob to move the carriage 44 along the track 46. The externallocation of a small portion of the capillary tube 32a and the casing ofthe transducer produces a relatively small, but appreciable, response tothe expansion state of the transducer fluid to ambient temperature, thusclosely compensating for changes in ambient temperature, which are inany event normally small in the environments in which such ovens areused.

For any given extension of the plunger 38, motion of the pivot 64 in thedirection of retraction of the plunger (leftward in FIG. 2) furtheropens the slots by compression of the spring 62 and motion of the pivot64 in the direction of further extension of the plunger 38 (rightward'in FIG. 2) closes the slots, if change of setting is made after anequilibrium such as shown in FIG. 2 already exists. If change ofsettingis made inthe cold condition, the temperature at which the slotswill start to open is altered by the resetting, but the opening does notcommence, at any temperature setting, until there is reached an oventemperature only slightly below the set temperature. For each presettemperature, equilibrium is of course reached at a slightly differentcondition of openness of the vents or slots i.e., a slightly differentposition of the slide-plate 52, but this difference is very smallcompared to the difference in positions of the carriage 44 whichcorrespond to the respective equilibrium temperatures.

There is shown in FIGS. 3 through 6, in greater detail, an improved andpreferred construction for the control assembly 34 of the oven system ofFIG. 1. The bellows assembly 36 with its plunger or piston 38 and theknob-and-dial 42 are as in the previous embodiment, and a similar screwmechanism 48 is employed in connection therewith, a large-diameter screw70 attached to the knob engaging an internally threaded sleeve 72affixed by bolts 74 to a plate 76 secured to the end of the carriage 78.The carriage comprises a plate 80 having slide-shoes 82, of nylon or thelike, on laterally opposite edges, the latter being slideable in opposedrecesses formed in a shaped sheetmetal track 84 bolted at 86 to aninternal bracket 88 extending between the ends of the interior of thehousing 90.

On the underside of the sliding plate 80 are spaced depending webs 92which are affixed at one end to the plate 76, and between which extend,at the opposite end, the pivot pin 94 of a lever 96. The leverextendsgenerally horizontally but has an upward extension tab or striker98 at the pivoted end, in the path of the plunger 38. A spring 100compressed between the housing 90 and ears 102 on the carriage 78prevents backlash of the screw mechanism 48, assuring accuratereproducibility of carriage position with dial setting.

A variable vent generally indicated at 104 is formed of a stationaryinner portion 106 and a reciprocable outer portion 108. The stationaryportion is in the form of radially spaced concentric rings 110maintained in this relative position by four radial webs 112,. thisassembly being mounted by screws 114 in a large aperture in the bottomof the housing 90. The reciprocable portion 108 is similarly formed ofconcentric rings 1 16, maintained in relative position by radial webs118. The reciprocable assembly is centrally secured at to a drive-rod122 which extends up through the axis of the stationary portion 106 andhas mounted at its upper end a yoke 124 bearing a roller-wheel 126 whichcontacts the lever 96. A compression spring 128 urges the rod 122upward, acting between the corners of the yoke 124 and a shoulderednylon bearing-sleeve 130 lining the hub of the stationary portion 106for free sliding motion of the drive rod 122.

In general, the operation of the control of FIGS. 3 through 6 is similarto that of the more elementary form of FIG. 2 already described.I-Iowever substantial advantages are obtained. The concentric rings 110of the stationary portion are offset radially from the concentric rings116 of the reciprocable portion, the latter being opposed to, and ofradial thickness only slightly greater than, the radial width of theannular spaces between the rings 110. In the closed position (not shown)a substantially entire blockage of air is readily achieved withreasonably smooth surface finishes at the contacting edges of the rings.The rate of change of air resistance with inward or outward motion ofthe reciprocable portion 108 is extremely high in the region adjacent tocomplete closure, so that very small motion produces a large variationof oven air intake. This variable vent structure, in addition, has verysmall resistance to air-flow for the area occupied when the vent is onlymoderately opened in position. These operational advantages are obtainedwith structure which is relatively simple and inexpensive to fabricate,as well as reliable and trouble-free in operation. For comparablesensitivity to motion, a sliding-surface construction like that of FIG.2 will be seen to require a very large number of very small apertures inthe movable and stationary parts of the vent. Such a construction is notonly difficult and expensive to fabricate, but may be shown to beincapable of reaching the low values of unit-area airflow resistancewhich can be obtained where the motion producing the variation in airadmission is reciprocating motion in the direction perpendicular to theabutting apertures surfaces, rather than motion in which one aperturedmember slides on the other. In addition, the variable vent structure ofFIGS. 3 through 6 requires very little attention by way of cleaning orother maintenance, the self-cleaning action of highvelocity air flowpreventing any accumulation of dust or the like.

In the closed position of the vent, the lever 96 is substantiallyparallel with the path of motion of the plunger 38, which is in turnsubstantially parallel with the track 84 along which the carriage 78 ismoved for temperature adjustment. Since the roller 126 supports thelever 96 in this parallel condition when the vent is closed, temperatureadjustment prior to start-up does not alter the rotational position ofthe lever 96 about its pivot 94. Accordingly (as is not the case in thestructure of FIG. 2) the point at which the plunger 38 engages the tabor striker 98 to commence opening of the vent after start-up varies inexact correspondence with the position of the carriage, i.e., thedistance of advancement or retraction of the carriage producessubstantially exactly the same difference in extension of the plunger 38at which opening of the vent commences. Since plunger extension isessentially linear with temperature, and since the further advancementof the plunger after opening commences until equilibrium is reached isrelatively negligible as compared with the plunger travel prior toengagement of the striker, temperature calibration of the dial 42 isfree of non-linearities which can arise from this source where theposition of the lever with respect to its supporting carriage issubstantially altered by the setting of temperature in the coldcondition of the oven, as in FIG. 2.

The manner of overall operation and the advantages of the oven systemillustrated in the drawing will be obvious from the earlier generaldescription of the invention, without necessity for repetition. In oneconstruction, a two-horsepower centrifugal blower was employed in awell-insulated and well-sealed cabinet-type oven. With the vents fullyclosed (i.e., the venting control disabled), a temperature of 235 C. wasobtained. Closely controlled temperatures up to 210 C. were preset andmaintained. The lower end of the controlled-temperature range isdetermined largely by the freedom of air-flow in the maximum openposition of the vent. With a sliding-plate construction such as that ofFIG. 2, the air-flow in the fully open position remained sufficientlyrestricted so that the minimum temperature obtainable was, with theparticular oven construction, 38 C. under ordinary room-temperatureexternal conditions. With the variable vent construction shown in FIGS.3 through 6, however, although of generally the same cross-sectionalarea, the resistance to air-flow in the open condition was sufiicientlysmall to permit close approach to room-temperature at the lower end ofthe calibrated scale of the temperature control.

It will be observed that in addition to the advantages earlier stated,the invention provides safeguards against hazards arising from blowerfailure. Where a blower is employed in a construction where the air isseparately heated, cessation of operation of the blower results inabnormally high temperatures particularly in the region of theelectrical or other heating elements. This hazard may of course beeliminated by provision for sensing the cessation of blower operationand shutting off the ure inherently eliminating the heat source. Thissafety aspect makes the invention highly advantageous even for useswhere explosion or other excessivetemperature hazards do not result fromemission of fumes or vapors by the load, but are inherent in the loaditself, for example in testing or processing of sealed containers ofexplosive material such as ammunition, cartridges of solid fuel, and thelike. If so desired, the blower speed may be selected to produce anypreselected maximum temperature even in the case of complete failure orblocking of the venting.

Many alterations and variations of the illustrated overall ovenconstruction will readily be devised. For example, for applicationshighly critical as to composition or purity of the oven atmosphere, orfor uses in contaminated environments, intake or surroundingenvironmental air may be undesired, and the air intake may be from asuitable sealed container of air (the broad meaning of which term asherein used has already been referred to) which is accordinglyequivalent to ambient air for this purpose. As another example, it maybe desired to produce relatively high temperatures in a work-chamberwith only moderate internal air velocity, as where the work-loadconsists of exposed powders or the like. In such a case, there may beprovided an auxiliary air-path within the oven by-passing the workchamber for permitting the use of a very highcapacity blower producingthe requisite rate of heat input while flowing only a small fraction ofthe recirculated air through the workchamber. Such a constructionpermits the obtaining of temperatures of the order of 300 C. to 500C.with only moderate air velocities in the work-chamber.

Although the invention is of greatest utility, as regards safety fromexplosion and the like, where no heating means are present which cancreate a hazard due to carelessness of an operator or malfunction of theoven equipment, the addition of auxiliary heating means is permissiblewhere such risk is acceptable. For example, if it is desired to speedthe reaching of equilibrium temperature in start-up, the heating by theblower may be temporarily supplemented by electrical heating elements inthis portion of the operation, just as is now conventional for similarpurposes where relatively small electrical heating elements are employedfor maintaining preset temperature once reached, the axuiliary heatingelements being disabled manually or automatically once their functionhas been served.

It will likewise be seen that the variable vent structure may, if sodesired, be employed for control of temperature even where the'bloweroperation is supplemented by additional heating means at all times, andfurther that many modifications of this portion of the structure may bemade without departing from this aspect of the invention.

Accordingly, the scope of the protection to be afforded the inventionshould not be limited to the particular embodiments illustrated anddescribed herein, but should extend to all use of the teachings of theinvention, as defined in the appended claims.

What is claimed is:

1. In a temperature-controlled oven comprising, in

combination:

a. an outer thermal-insulation leakage-sealed housing,

b. an inner chamber within the housing adapted to receive bodies to bemaintained at constant temperature and having air-inlet and air-outletportions,

c. air-recirculation means within the housing adjacent to the innerchamber including a constantspeed air-blower and air-duct means guidingoutlet air from the chamber back to the inlet portion, and

d. an air inlet vent and an air outlet vent through the housing inregions of differing internal air pressure, con-tinuously blendingambient air into the recirculated air, the housing being sealed againstleakage,

the improvement for safety against explosion and like .hazardscharacterized by:

e. the sole source of heat being the heating of the air by the blower,and

f. means on at least one of said vents responsive to the oventemperature to vary the rate of blending of ambient air. I

2. The improved oven of claim 1 also having means responsive to ambienttemperature to vary the rate of blending of ambient air, the change ofblending-rate with change of ambient temperature being small compared tothe change of blending-rate with change of oven temperature.

3. The improved oven of claim 2 wherein the blending-rate varying meanscomprises a thermal-expansion temperature transducer having a portionexposed to oven air and a portion exposed to ambient air and an actuatorpositioned in response to the temperature transducer.

4. The improved oven of claim 3 wherein the varying means comprises aspring maintaining the air-inlet vent in a normally closed position, alever coupled to the actuator and the vent to open the vent in responseto actuation by the actuator, and manually operable means to set therelative position of the transducer and the lever to thus set the oventemperature.

5. A method of producing preselected temperature in an oven comprisingcontinuously recirculating the oven air within the oven by means of ablower while heating the air substantially entirely by the interactionof the blower with the air, continuously blending a small amount ofambient air with the oven air, and continuously varying theblending-rate in response to changes in oven temperature.

6. The method of claim 5 further characterized by additionally varyingthe blending-rate in response to changes in ambient temperature, thevariation of blending-rate with change of ambient temperature beingsmall compared to the change of blending-rate with change of oventemperature.

1. In a temperature-controlled oven comprising, in combination: a. anouter thermal-insulation leakage-sealed housing, b. an inner chamberwithin the housing adapted to receive bodies to be maintained atconstant temperature and having air-inlet and air-outlet portions, c.air-recirculation means within the housing adjacent to the inner chamberincluding a constant-speed air-blower and airduct means guiding outletair from the chamber back to the inlet portion, and d. an air inlet ventand an air outlet vent through the housing in regions of differinginternal air pressure, con-tinuously blending ambient air into therecirculated air, the housing being sealed against leakage, theimprovement for safety against explosion and like hazards characterizedby: e. the sole source of heat being the heating of the air by theblower, and f. means on at least one of said vents responsive to theoven temperature to vary the rate of blending of ambient air.
 2. Theimproved oven of claim 1 also having means responsive to ambienttemperature to vary the rate of blending of ambient air, the change ofblending-rate with change of ambient temperature being small compared tothe change of blending-rate with change of oven temperature.
 3. Theimproved oven of claim 2 wherein the blending-rate varying meanscomprises a thermal-expansion temperature transducer having a portionexposed to oven air and a portion exposed to ambient air and an actuatorpositioned in response to the temperature transducer.
 4. The improvedoven of claim 3 wherein the varying means comprises a spring maintainingthe air-inlet vent in a normally closed position, a lever coupled to theactuator and the vent to open the vent in response to actuation by theactuator, and manually operable means to set the relative position ofthe transducer and the lever to thus set the oven temperature.
 5. Amethod of producing preselected temperature in an oven comprisingcontinuously recirculating the oven air within the oven by means of ablower while heating the air substantially entirely by the interactionof the blower with the air, continuously blending a small amount ofambient air with the oven air, and continuously varying theblending-rate in response to changes in oven temperature.
 6. The methodof claim 5 further characterized by additionally varying theblending-rate in response to changes in ambient temperature, thevariation of blending-rate with change of ambient temperature beingsmall compared to the change of blending-rate with change of oventemperature.